Avoiding casing damage during direct steam drive oil production



United States Patent O 3,420,298 AVOIDING CASING DAMAGE DURING DIRECT STEAM DRIVE OIL PRODUCTION Archie J. Cornelius, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Aug. 4, 1967, Ser. No. 658,315

U.S. Cl. 16611 8 Claims Int. Cl. E21b 43/24 ABSTRACT OF THE DISCLOSURE Oil is produced from an oil stratum by a steam drive from an injection well to one or more offset production wells by periodically substantially reducing the rate of steam injection during a soak period to allow heat transfer to the stratum oil, the steam rate being reduced to that sufficient to merely maintain established casing temperature during the primary injection phase so as to avoid contraction and expansion of the casing in the injection well with attendant cratering or collapsing.

This invention relates to a process for producing oil from an oil stratum with a direct steam drive while avoiding casing damage in the injection well.

The production of oil from an oil stratum by a direct steam drive from an injection Well to one or more offset production wells is a conventional process in the petroleum production industry. The process generally is continued until steam breaks through at the production wells. Water has conventionally been injected either before or after steam breakthru in order to fill up the reservoir volume which has been occupied by steam, preventing reservoir fluids from entering the steam-swept zone.

A more recently developed process known as huffpuff comprises injecting steam for a substantial period, terminating the injection of steam for another substantial period with the injection well shut in, and then opening the injection well to flow for a substantial period. In the huff-puff process, either the surrounding production wells are shut in or steam is not injected for a sufiicient period to drive the oil to the surrounding Wells. In the huff-puff process, the soak period in which steam is not injected thru the well allows the casing in the injection well to cool substantially by heat transfer to the surrounding earth structure and the casing is again heated to the approximate temperature of the injected steam during the resumption of steam injection. This expansion and contraction of the casing string in the injection well has been found to result in cratering or collapsing with attendant cracking and parting of the threads at the joints of the casing.

The present invention is concerned with a steam drive process which is more effective in producing oil and which avoids casing damage resulting from the huff-puff technique of steam drive.

Accordingly, it is an object of the invention to provide an improved process for producing oil from an oil stratum by a steam drive. Another object is to provide a modified huff-puff steam drive process which avoids casing damage in the steam injection well. A further object is to provide a combination direct drive and huffputf steam injection process in the recovery of oil. Other objects of the invention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

A broad aspect of the invention com-prises injecting steam thru a cased injection well into an oil stratum until steam or steam condensate is produced in one or more offset production Wells, then, during a soak period, substantially reducing the rate of steam injection to a rate 3,420,298 Patented Jan. 7, 1969 sufficient to merely maintain the established casing temperature in which heat from the previously injected steam flows into the oil adjacent the steam injected area, and following the soak period again injecting steam at the usual high rate. Oil flows into the surrounding production wells during the soak period as well as during the steam injection period using high rates of injection. Oil is not produced thru the steam injection well in any substantial quantity either during the regular steam injection phase or the soak phase of the operation as in the huff-puff technique of production.

Steam is injected into the stratum thru the injection well at a temperature in the range of about 500 to 750 F, preferably in the range of about 500 to 600 F. for a period in the range of about one to six months followed by a soak period in the same range during which the steam injection is maintained at a relatively low rate sufficient to merely maintain the established casing temperature during the principal steam injection period. Usually the steam injection rate during the soak period is in the range of about one third to one tenth of the steam injection rate used in the principal steam injection phase of the process. The primary steam injection rate is usually in the range of about 200 to 2000 pounds of steam per hour per foot of oil stratum thickness penetrated by the injection well. A steam pressure in the range of about 1000 to 2500 psi. is generally utilized in the steam injection well.

In most oil strata, the permeability of the oil sand varies at different levels so that there are relatively high permeability streaks running between an injection well and a ring of surrounding production wells. When steam drive is practiced in a stratum of this character, steam or hot condensate breaks thru into the production wells without much increase in oil production. By terminating rapid steam injection when breakthru occurs and allowing a substantial soak period for heat to migrate into the adjacent oil stratum from the injected steam within the high permeability streaks or fingers leading to the production well, the viscosity of the heated oil is substantially reduced so that the more fluid oil flows into the fingers or streaks during the soak phase of the operation. The ensuing rapid steam injection phase of the operation then produces the oil from the fingers during the next phase of operation.

In some instances, the injected steam is passed directly into the casing at the wellhead without utilization of a tubing string, While in other applications, steam is injected at the well head into a tubing string extending substantially to the oil stratum being produced. In either event, without the use of the invention, the casing is heated substantially to the temperature of the injected steam and termination of steam injection for an extended soak period followed by a substantial steam injection period at high rates greatly cools the casing and again heats it to steam injection temperatures with resultant undue stress and possible damage to the casing string. The steam injection and soak phases of the operation are at least one week in extent and usually in the range of one to six months so that ample time is provided for the damaging change in temperature of the casing string without the use of the technique of the instant invention.

As illustrative of the invention without unduly limiting the same, the process was applied to a four-spot well pattern (a central well and three ring wells) in an oil stratum '60 to 70 feet in thickness at a depth of about 3200 feet and containing a heavy oil having an API gravity of 10 to 12. Steam was injected at a temperature of about 572 F. and a pressure of about 1250 p.s.i. at the rate of 20,000 ponds per hour for a period of about four months at the end of which there was breakthru of steam into the production wells. The rate of injection steam was cut back to about 5000 pounds per hour and this low injection rate was continued for a period of approximately two months before again injecting steam at the high rate of 20,000 pounds per hour. The casing string was heated to a temperature of approximately 572 F. during the high rate of steam injection phase of the process and during the soak period the lower steam injection rate maintained the casing temperature substantially at the established temperature during the first phase of operation.

Thus, operation in accordance with the invention protects the casing string from alternate expansion and contraction during the steam injection and soak phases of the operation. It also results in enhanced oil production due to the flowing of less viscous oil into the flow path of the steam during the soak period, resulting from migration of heat into the adjacent stratum and condensation of the steam which has the effect of drawing the more fluid oil into the condensation area.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. In a process for producing oil from an oil stratum penetrated by a cased steam injection Well and one or more offset production wells wherein steam is injected for a substantial period of at least a week to produce oil in said production wells and the injection of steam is terminated for a substantial soak period of at least a week and thereafter steam injection is resumed, to permit heat to soak from the injected steam into the adjacent areas of stratum before resuming steam injection and wherein the casing in said injection well cools and contracts during the soak period and again heats and expands during a subsequent steam injection period, causing excessive stress on the casing, the improvement comprising injecting steam thru said injection well at a substantially reduced rate during the soak period so as to prevent any substantial reduction in the casing temperature, thereby avoiding excessive casing stress due to alternate contraction and expansion thereof.

2. In the process of claim 1 injecting steam thru said injection well for a period in the range of about 1 to 6 months followed by a soak period in said range in which the steam injection rate is reduced to a rate which merely maintains casing temperature established during the high steam injection phase preceding the soak phase.

3*. In the process of claim 1, injecting steam prior to the soak period until steam or condensate is produced in at least one of said production wells before reducing the rate of steam injection in the soak period.

4. In the process of claim 1, injecting said steam directly into the casing free of a tubing string.

5. In the process of claim 1, injecting said steam into said injection well thru a tubing extending substantially to said oil stratum.

-6. In the process of claim 1, using a steam injection rate in the range of about 200 to 2000 pounds of steam per hour per foot of oil stratum thickness.

7. In the process of claim 6, injecting steam at a temperature in the range of about 500 to 600 F., a pressure in the range of about 1000 to 2500 p.s.i., and until breakthru of steam or hot condensate in one of said production Wells and then reducing the steam injection rate to substantially maintain established casing temperature.

8. In the process of claim 1 wherein said oil stratum contains horizontal streaks of relatively high permeability and injected steam breaks thru into a production well with only minor increase in oil production, decreasing the rate of steam injection substantially at said .breakthru to substantially maintain the established casing temperature.

References Cited UNITED STATES PATENTS 3,259,186 7/1966 Dietz 16640 X 3,330,353 7/1967 Flohr 166-40 3,333,637 8/1967 Prats 16640 3,349,849 10/1967 Closmann 166-40 3,354,958 11/1967 Parker 16640 3,358,762 12/1967 Clossmann 16640 3,369,604 2/1968 Black et a1 166-40 3,367,419 2/ 1968 Van Lookeren 16640 X STEPHEN NOVOSAD, Primary Examiner.

U.S. Cl. X.R. 166-40 

